Backups prevent loss of data by ensuring data can be recovered.
Backup and recovery go together: backups are not taken to not support a recovery, and recovery cannot be completed without having a backup from which to recover. For this reason, many methods focus on the recovery side (as that is the most vital part of any backup scheme).
There are several backup methods available, ranging from bare-metal backup (and recovery) to record-based backups in a database system. Backup solutions offer a number of different features such as backup snapshots based on time, deduplication, the ability to recover single modified files or whole directory trees, etc.
Bare metal recovery
In case of bare-metal recovery, software is used that is not installed on the operating system that is under backup/recovery. The result of a bare-metal restore is a fully bootable system again.
Most of these recovery solutions are based on partition imaging (like with CloneZilla, PartImage, or FSArchiver), although in Gentoo, stage4 snapshots can also be used as some sort of bare-metal recovery solution.
File and directory recovery
For a more selective approach, a file- and directory-based backup/recovery model is used. For such situations, on-system software is responsible for regularly taking copies (or patches/diffs) from a predefined list of files and directories. Many solutions exist, such as Bacula or BackupPC, but simple schemes can also be obtain by properly using rsync or just plain copies.
Some applications offer a more specific approach on backup and restores. Databases are a prime example (as their job is to guard over data) but others, like version control systems, often have specific backup/restore routines too.
When hosting one or more services, it is wise to look at the backup/restore routines for each service and implement them on top of the other backup schemes.
A few principles need to be closely guarded when implementing backups:
- Always verify that the backups can be used to restore. Either restore to another location or system, or restore immediately after taking a backup. Too often users forget this and are severely disappointed when they find out that their daily backups didn't do much (captured the wrong directory) or cannot be restored.
- Keep backups on a safe location. Try to have them off premises. Move them regularly to a family members home, or send them over the Internet to a cloud storage provider - password-based encryption schemes can be used to protect confidentiality with off site backups.
- Mix backup methods. Take a full system (bare-metal) backup once in a while, with file and directory backups more regularly and application-level backups as much as possible (since that is most likely what clients/users will be most angry about if they lose it).
- Mirroring is not having a backup. Mirroring keeps two sides in sync, whereas a backup is a snapshot of data at a point in time.